Since the application of electric arc discharges between an anode and a cathode as vaporization sources or as a means to generate ions of solids, it has been desirable to confine the vapor and the ion emission to a definite region of the cathode, specifically, the region from which the emitted molecules of vapor or ions can be optimally utilized. Because the material to be evaporated, or ionized, only exists in suitable composition for an intended application in this particular region; (we may think of the vacuum metallization of alloys, for example) the emission of the particles for geometrical considerations should only obtain from a definite region of the cathode, in order to deposit the substrates only under certain directions of incidence, for example, or to prevent the exposure of certain parts of the facility in a deposition layout. Finally, in each instance it is necessary to make sure that the electric arc cannot extend far enough at the rim of the cathode to damage the cathode mounting or other parts of the facility. This has represented a problem, especially when a component that is electrically positive relative to the cathode or that is entirely connected to the anode potential and was situated in the vicinity of the cathode. This case is the so-called tracing point of the arc discharge, which normally wanders in fits and starts back and forth from one point to another on the cathode surface. The tracing point might be stabilized in the proximity of such component and heat the latter too intensely by virtue of too long a burning time at the particular point, so that the component or the cathode mounting would be damaged. In order to deter this, various measures have already been proposed to confine the motion of the tracing point of an arc discharge to a preselected region of the cathode surface. However, these measures currently have brought only some success in cases when the discharge produces only a single tracing point at not too high a current intensity. But if several tracing points begin to form in the case of more intense discharges of around 50 amperes or more of total current intensity (which on the basis of the present experience cannot be avoided), the problem can no longer be mastered by the known measures.
In British patent No. 1,322,670 and the corresponding U.S. Pat. No. 3,793,179, it is proposed to provide a means of extinguishing the tracing point of an arc discharge as soon as the latter emerges from the permissible region of the cathode surface. For this purpose, shielding plates were arranged as close as possible, e.g., at a distance of a couple of millimeters, in front of the portions of surfaces to be protected against the discharge, which surrounded the outer perimeter of the cathode in the form of ring. Evidently, these rings could be made of any desired material, that is, either an electrically conducting material or an insulator. To be sure, only shields of magnetizable material, and especially a soft magnetic material, such as an appropriate steel, were recommended for use, as it was purportedly observed that use of a shield of different material unfavorably influenced the stability of the arc discharge.
In this respect, (10 years after publication of the aforesaid proposal, by a different party German patent No. 33 45 442), it was again recommended to employ a limiting ring of magnetically permeable (soft magnetic) material, in particular, one such as soft iron or the alloy Permalloy, for the purpose of limiting the cathode spot on the target surface in a device for stabilization of a vaporization arc. This was supposed to deflect the arc back to the target surface whenever it approached the limiting ring. Other materials said to be suitable for such limiting ring, besides iron, were nickel and cobalt or alloys thereof. But the important point, as stressed, was that the limiting ring should be magnetically permeable, in order to hold the arc on the target surface by virtue of this property.
All of these familiar measures for preventing the arc from wandering out of a permitted region of the cathode surface were sufficient (particularly) for an arc with only one tracing point of the discharge. But at overall current intensity of the arc approximately 50 amperes or higher, multiple tracing points (cathode points, "cathode spots") form in increasing measure, and experience reveals that the materials hitherto recommended as the optimal solution for the limiting rings fail in this case. In fact, it was found that, when several discharge tracing points occur, the arc can abruptly occupy a position outside the permitted cathode region (i.e., no longer wander back and forth on the cathode surface), so that damage may occur in this location, in particular, burnthrough of the cathode and its cooled supporting base.
The reason for this statistically occurring defect is not entirely clear, but evidently a sizable portion of the total arc current intensity is concentrated at this instant on a tracing point located in a forbidden position, whereupon the limiting ring can no longer perform its intended function, i.e., it is no longer able to turn away the arc from touching the margin.
In any case, it is continually observed that, when the current intensity is greater than 50 amperes, despite the partitioning of the total current into several separate arcs which occur, and which would lead one to suppose that the individual discharge branch does not carry too large a current density, nevertheless damages occurs that could still have been prevented by the previously known magnetizable limiting rings of soft magnetic material in the case of discharges with only one tracing point.
In the quest for additional possibilities of stabilizing an evaporation arc, German patent 33 45 493 propounded the use of a limiting ring that is in contact with the target and surrounds the target surface, while this limiting ring should consist of a material having a secondary electron emission ratio smaller than 1 for average energies of the charged particles of the arc, and in order to confine the cathode spot on the target surface it should have a surface energy smaller than that of the evaporated target material. In particular, it was proposed to use materials containing nitride compounds for the limiting rings, and in another version, a ring consisting of iron or Permalloy, in which case a layer with appropriate secondary electron emission ratio should also be deposited on the magnetically permeable ring, if need be.
In the case of a magnetically permeable ring, it had been observed that (if the target material itself is non-magnetic, at any rate), the arc wandering in random motions about the target surface dwells on one particular spot for approximately one second and then moves on to other zones of the cathode. From German patent 33 45 442, it can be deduced that limiting rings of magnetically permeable material should be used because, owing to their magnetic properties, they can hold the arc on a non-magnetic target. It was believed that a magnetically permeable material would be needed in order to create a sufficiently strong magnetic field by the discharge current.
The purpose of the limiting rings, as mentioned, is to confine the apparently aimless motion of the cathode points, which are the source of the particle emission in an arc discharge on a cold cathode, to the intended surface of the cathode. The ring of magnetizable material, heretofore regarded as the best solution, achieves this for discharges with only one tracing point, although the reliability with which it performs this function is no longer satisfactory when the cathode is operated at a higher arc current, so that several tracing points exist at the same time. Contrary to the prevailing view on the action of a magnetic field, this reliability is even further reduced if a magnetic field is provided normal to the cathode surface, in order for example to create suitable conditions for deposition of a layer on a substrate. Even if the arc current intensity is only moderately raised, so that only a few cathode points move simultaneously over the cathode surface, breakout of a cathode point takes place after a somewhat prolonged on-time, in any case, after several hours of operation. This may then move into disallowed regions, and propitious burn conditions will exist, for example in the neighborhood of insulators or other structural components, thereby causing damage. Hence, not even an occasional abandonment of the designated cathode surface can be tolerated in industrial operation.